Recently, on-axis electrode geometrical aberration corrector units were reported in the context of ring-cathode electron/ion microscope columns. These on-axis electrode geometrical aberration corrector units were designed for the purpose of cancelling second-order off-axis geometric aberrations in the column, helping to match both off-axis aberrations from the ring-cathode gun lens as well as those from the objective lens.
In a conventional scanning electron/ion microscope, the electric or magnetic lenses that demagnify the primary beam are rotationally symmetric. FIG. 1 shows a schematic side view of a final stage 100 of a conventional electron microscope (e.g., the final stage of a conventional probe forming in an electron/ion microscope). After the primary electron beam 110 emerges from a crossover point 102, typically located mid-way down the column 112, the electron beam 110 travels through a hole aperture 104, and is then focused by an objective lens 106 to a (focal) point 108 (e.g., on a specimen), usually referred to as the beam probe 108 or the electron beam probe 108.
The spatial resolution of scanning electron/ion microscopes may be limited by several objective lens aberrations which determine the smallest possible probe size that can be formed. Typically, there are three main types of aberrations: spherical (S) aberration which is created by variations in angle within the electron beam when traveling through the lens (which, for example, may often dominate in large aperture or long working distance), chromatic (C) aberration which is the beam's inherent energy spread (which, for example, may often dominate at low beam voltage), and diffraction (D) aberration which is generated by the intrinsic wave nature.
The angles at the probe 108 may range from zero to a maximum semi-angle, θ, as shown in FIG. 1, which may scale directly with the aperture radius of the hole aperture 104. All objective lens aberrations (e.g., on-axis aberrations) are functions of the maximum semi-angle θ. Of all three objective lens on-axis aberrations, it is spherical aberration that grows most rapidly with the semi-angle, e.g., the radius of the spherical aberration is proportional to θ3, while chromatic aberration increases linearly with θ, and diffraction aberration varies inversely proportional with θ.
The probe current, another fundamental parameter of the scanning electron/ion microscopes, may be determined by the transmission area (TM) of the aperture. For example, the probe current is directly proportional to the transmission area of the aperture 104 (e.g., proportional to the square of the aperture radius) and is important for good signal-to-noise ratio (SNR) performance. A higher probe current may be obtained by enlarging the aperture radius. However, this comes at the expense of increasing the radius of the spherical aberration. In practice, in order to minimize the final probe diameter, an optimum semi-angle θ (aperture radius) may be used. The conventional scanning electron microscope is therefore not able to combine high probe current with high spatial resolution, as one must be sacrificed at the expense of the other.
In one example, a charged particle beam corrector and charged particle beam apparatus have been found to have an ability to correct spherical aberration by creating magnetic field by current carrying wires. However, such apparatus increases the chromatic aberration and is not rotationally symmetric, resulting in the impracticality of the apparatus.
In another example, a charged particle beam trajectory corrector and charged particle beam apparatus have been found to have an ability to correct spherical aberration by electric fields. However, this apparatus increases the chromatic aberration.
Thus, there is a need for a mechanism to address at least the above-mentioned problems that uses both magnetic and electric field corrector units to compensate for spherical aberration and/or chromatic aberration, while obtaining high probe current and high spatial resolution at the same time.